1. Field of the Invention
The present invention relates to a communication technology, and it particularly relates to a method for transmitting a packet signal composed of multiple subcarriers, and a radio apparatus and a communication system utilizing the same.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation schexme has been applied to the wireless LAN (Local Area Network) standards such as IEEE 802.11 a/g and HIPERLAN/2. The packet signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carries in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the packet signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, Sept. 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and packet signals to be transmitted in parallel are set (hereinafter, each of data and the like to be transmitted in parallel in a packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate. In the MIMO system, the data rate can also be adjusted by increasing or decreasing the number of antennas to be used for data communications. Furthermore, the data rate can be adjusted in greater detail by applying the adaptive modulation to the MIMO system. To perform such an adjustment of data rates more reliably, the transmitting apparatus should acquire from the receiving apparatus the information on data rates suited for the radio channel between the transmitting apparatus and the receiving apparatus (hereinafter this information will be referred to as “rate information”). To raise the accuracy of the rate information like this, it is desired that the channel characteristics between a plurality of antennas in the transmitting apparatus and a plurality of antennas contained in the receiving apparatus, respectively, be acquired by the receiving apparatus.
To improve the accuracy of rate information in the above-mentioned requirements, it is necessary that the channel characteristics be acquired with high accuracy. To improve the accuracy in the acquisition of channel characteristics, the transmitting apparatus or the receiving apparatus transmits from all of antennas the known signals for use in channel estimation. Hereinafter, the known signals, for use in channel estimation, assigned to a plurality of streams will be referred to as “training signals” independently of the number of streams to which data are assigned. For example, even though data are assigned to two streams, the training signals are assigned to four streams.
Under these circumstances, the inventor of the present invention came to recognize the following problems to be solved. When the training signals are transmitted, the number of streams containing known signals for use in channel estimation (hereinafter referred to as “channel estimation known signals”) differs from that containing data. A known signal for setting AGC (Automatic Gain Control), hereinafter referred to as “AGC known signal”, at the receiving side is assigned anterior to the channel estimation known signals. When an AGC known signal is assigned only to a stream where data is assigned, part of the channel estimation known signals is received, in a state where the AGC known signal has not been received, anterior thereto. In particular, when the strength of AGC known signal gets larger at the receiving side, the gain of AGC is set to a value which is large to a certain degree. In so doing, when the strength of channel estimation known signal of a stream where the AGC known signal is not assigned is larger, there is a strong possibility that said channel estimation known signal may be amplified to such a degree that distortion is caused by AGC. As a result thereof, the error in channel estimation based on said channel estimation known signal becomes large.
On the other hand, when an AGC known signal is assigned to a stream where a channel estimation known signal is assigned, the number of streams to which the AGC known signal is assigned differs from the number of streams to which data is assigned. Hence, there is a possibility that the gain set by the AGC known signal is not suitable for the demodulation of data. As a result, the demodulated data are subject to errors. In particular, if the difference between the number of streams to which the channel estimation known signals are assigned and that to which the data are assigned gets larger, these problems to be solved will become more important issues.
When the base station apparatus multiplexes the communication with a plurality of terminal apparatuses, CSMA (Carrier Sense Multiple Access) is carried out. For the purpose of improving the transmission efficiency, one basestation apparatus occupies a frequency band in a predetermined period of time so as to continuously transmit a plurality of packet signals. That is, the base station apparatus specifies, in partial periods of time, the timing at which the signals are to be transmitted to a plurality of terminal apparatuses (hereinafter referred to as “transmit timing”) and the timing at which the signals from a plurality of terminal-apparatuses are to be received (hereinafter referred to as “receive timing”). Then the base station apparatus informs respectively the plurality of terminal apparatuses of said specification, and each of the plurality of terminal apparatuses carries out a processing in accordance with said specification (hereinafter, such a processing will be referred to as “assignment mode”). Here it is assumed that after a plurality of transmit timings for the plurality of terminals are specified, a plurality of transmit timings are specified consecutively. A terminal apparatus receives a signal at the specified transmit timing. Even if the aforementioned training signals are transmitted under such circumstances, it is still desirable that the degradation of transmission efficiency be prevented.